Results of investigations having the purpose to correlate Partial Discharge (PD) patterns to specific defect typologies that can affect the GVPI insulation of MV stator windings, are summarized in this paper. It was found that some defects can generate patterns with different shapes (multiple or variable patterns) as well as patterns having the same shape can be due to different causes (ambiguous patterns). These results were compared with those recorded testing bars of HV generators manufactured with resin-rich insulation technology and adopted as reference patterns in current guides or technical specifications. Comparison of PD patterns resulting from the degradation of components manufactured using different insulation technologies e.g., taped or varnished stress grading, shows different features. These results clearly show that each insulation technology requires a specific set of PD patterns while additional rules are necessary to identify patterns whose root cause can be misleading.
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